1. Field of the Invention
The invention relates to microfiltration and to ultrafiltration using membranes subjected to a tangential flow.
The term microfiltration is usually applied to particle sizes of a suspension between a few hundredths of micrometers and a few tens of micrometers and is carried out for example at pressures between a few tens of bars and a few bars, whereas ultrafiltration aims at separating the large organic molecules from mineral molecules or from small organic molecules and takes place for example at pressures between 2 and 15 bars.
2. Description of the Prior Art
Causing the prefilt to flow at the surface of the membrane, (tangenetial flow), at speeds of the order of a few meters/second, ensures the permanent elimination of solids which tend to be deposited on the filtering surface, as well as the material inputs and renewals.
However, the efficiency of the known technique of tangential flow is not always satisfactory when it is considered for industrial applications, particularly for filtering liquids with a high solid material charge, even using a modular construction of the filter, that is to say forming it by a series and/or parallel assembly of modules each formed from a plurality of juxtaposed elementary cells, which allows the overall filtering surface to be obtained required for practical use by assembling together the relatively small surfaces of the elementary cells.
A sufficient yield of the membrane surface is in fact only obtained if a sufficient hydrodynamic homogeneity is provided, that is to say a good equal distribution both of the speeds and of the pressure gradients over the whole surface.
The known technique the most widely used in industrial tangential flow microfiltration uses a tubular mineral membrane formed of a rigid and porous mineral tube coated on the inside with an adjuvant film with ultrafine pores. Such elementary cells are assembled together in stacks so as to form a module. In order to obtain a large filtering surface for a given volume, a large number of practically contiguous tubes are grouped together in the stack, and the outlet for the filtrate from the central tubes is then braked by the resistance to the passage, so that a counterpressure is formed inside the corresponding cells, which adversely affects the hydrodynamic homogeneity, so the filtration yield.
The construction of such filters is moreover relatively expensive.
Other known types of elementary membrane filtering cells have either too great a thickness of the liquid stream flowing over the membrane (whence an insufficient speed gradient), or a linking up path for the filtered product which is tortuous and long and generates pressure losses which act counter pressurewise (impeding draining of the filtrate), or an inner lining of the membrane (inter membrane cloth) which also induces high pressure drops, or finally mechanical fragility.
By way of example, the membranes formed of a non woven textile web coated or impregnated with polymers present impeding draining of the filtrate and their open cellular structure leads to the irreversible trapping of the particles, the "sieve" membranes (polymer film which is subjected to bombardment with heavy ions) have a thickness limited to 10 .mu., which makes use thereof difficult; finally, the ultrafiltration membranes of the composition multilayer type have an active layer with toruous structure in which irreversible inclusions of particles may occur.
Depending both on the known structures of modules (with hollow fibers, tubular, spiral or flat) and on the types of membrane used by different coefficients of worth are obtained, which take into account the pressure drops in the filtrate or the concentrate, the energy consumption, the possibility of treating charged liquids, the ease of thickness adaptation. An important factor is also formed by the "dead" volume per square meter of membrane in the module, that is to say the volume which it is necessary to set in motion in order to obtain the efficient speed: the lower this volume, the greater will be the energy performance of the module.